Extensive efforts have been applied in the field of head positioning servo systems to developing improved techniques for ensuring that an active read/write head flies precisely over the centerline of a desired data track in order to minimize data read or data write errors and to further reduce track pitch in order to increase areal recording density. Improved techniques for increasing areal recording density have been an important enabling factor in the trend toward smaller yet higher capacity disk drives.
Areal recording density is generally expressed, analytically, in terms of the product of track density (the number of concentric tracks per radial inch) on the surface of a disk and the bit density (the number of bits per linear inch) that can be recorded along a given data track. As the track density increases, thereby reducing the spacing between tracks (track pitch), it is necessary to provide more precise radial positioning of an active read/write head over the centerline of the desired track.
Various types of head positioning servo systems have long been known in the field and pertinent such servo systems include a particular type, often referred to as an "embedded servo", in which each recording surface of a disk has particular arcuate locations termed "servo sectors" that are reserved for servo burst fields and that are not used for storing user data. During time spaced-apart intervals, in which a servo burst field is being read by an active head, the servo system has access to the information needed to determine whether or not the active read/write head is accurately positioned over the centerline of a desired data track. If it is determined that a position error exists, the servo system is able to effect an appropriate adjustment to the active read/write head to cause the active head to fly over the centerline of the desired track.
During operation of such an embedded servo system, an active read/write head flies above a servo sector and reads sequential servo burst fields during a sequence of time windows in order to produce a servo read signal. Signal processing circuitry responds to the servo read signal in order to produce periodic, time-varying signals that represent the amplitude of the servo read signal during the successive time windows. Additional processing circuity responds to the servo read signals to produce a servo error signal defining a magnitude and direction of any off-track error between the actual and the desired position of the read/write head. The servo error signal is used to drive a disk drive's head positioning actuator assembly which causes the head to move radially over the disk surface to thereby fly over the centerline of the desired track.
In the simplest case, commonly referred to as an "A-B" servo pattern, when the head flies exactly over the track centerline, a sequential reading of the oscillating, periodic "A" and "B" servo burst fields causes the servo system to produce a null servo error signal if the read/write head is ideally positioned over a track. The "A" servo burst read signal should have each of its positive and negative going oscillating peaks giving precisely the same magnitude value. With all such peaks having the same value, the "A" servo burst read signal would thereby define a uniform "burst amplitude". Follow-on signal processing circuitry is then able to properly demodulate the "A" servo burst read signal in order to produce a demodulated analog signal having a value corresponding to the uniform amplitude of the "A" burst. Likewise, the "B" servo burst read signal would have each of its peak values giving the same, consistent magnitude during the timing window in which the "B" burst is read. Follow-on signal processing circuitry would then properly demodulate the "B" burst in order to produce an analog signal having a value corresponding to the "B" burst's uniform amplitude. Under ideal circumstances, involving two demodulated signals having the same value, the difference between them, as represented by the servo error signal, would be null.
If the active read/write head is not precisely over the centerline of a particular track, the sequential reading of a servo "A-B" burst pattern will cause follow-on servo demodulation and processing circuitry to produce a servo error signal, indicating both the magnitude and direction of the read/write head's position error from the desired track centerline. For example, if a read/write head is positioned more towards the "B" burst portion of the track, the "B" burst will be read as having a higher amplitude than the "A" burst and, when the two bursts are demodulated and processed, the difference in amplitudes will indicate the degree by which the read/write head is off the track centerline, while the sign of the difference indicates the direction that the head is displaced relative to the track centerline. By comparing the values of the "A" and "B" demodulated signals, servo processing circuity can determine the transducer head position relative to the desired data track centerline.
Thus, it may be seen that the accuracy of a head positioning servo system depends greatly upon the accuracy of the servo error signal which, in turn, depends greatly upon the accuracy of the servo burst read signal processing circuity. As part of the process, an alternating polarity servo read signal, representing a servo burst, must first be amplified in order to define a signal suitable for processing by follow-on circuitry, and rectified in order to allow efficient processing by modern CMOS analog ADC/DAC circuitry. Once the signal is amplified and rectified, a demodulated analog signal, representing the burst amplitude is produced by determining the average value of the magnitudes of the peaks, i.e., a peak averaging approach, or by determining the area defined under each individual peak and taking the average of the areas or a normalized sum of all of the areas in order to represent the burst signal's amplitude, an approach termed area integration.
Referring now to FIGS. 1 and 2, there is depicted a generalized block diagram of a servo channel gain configuration according to the prior art and a generalized waveform diagram depicting a portion of an exemplary processed alternating polarity signal representing a servo burst at various stages of the amplification and rectification process. As shown in FIGS. 1 and 2, an in-coming representative servo read signal is an oscillatory, periodic, alternating polarity waveform which characteristically exhibits both positive-going and negative-going peak excursions. Conventionally, preamplifier circuitry comprising a typical read/write transducer head defines a read signal, whether servo or data, as a differential signal, with each signal having a 180 degree phase relationship with the other. One of the differential signals is conventionally termed V.sub.in+, or "positive"; the other, 180 degrees out-of-phase, is conventionally termed V.sub.in-, or "negative", such that positive excursions on the V.sub.in+ differential signal are mapped over negative excursions on the V.sub.in- differential signal. In FIG. 1, the positive and negative differential voltage signals are provided to a gain stage 10 at respective inputs 12 and 14. One of these differential signals, the positive or V.sub.in+ signal, is depicted in the exemplary timing and waveform diagram of FIG. 2. Since the positive and negative differential signals are identical except for their phase relationship, any description pertinent to the "positive" differential signal is equally applicable to the "negative" signal.
Differential input voltage signals are first amplified by a transconductance amplifier 16, by which the alternating polarity differential voltage signals are amplified and converted into alternating polarity differential current signals. As is customary practice in the art, the amplification factor of a transconductance amplifier is given as "gm", and is defined as the small signal gain developed through the device. As was described in the case of the input voltage, the current output of the transconductance amplifier 16 defines a double ended, differential signal comprising a "positive" current signal, denoted i+, and a "negative" current signal, denoted i-, having the same 180 degree phase relationship as the differential input signals. When evaluated against a zero reference, the amplified "positive" current waveform, output by the transconductance amplifier 16, would be as represented by the curve designated i+ in the exemplary waveform diagram of FIG. 2.
Following transconductance amplification, the differential, alternating polarity current signals are further amplified by a transimpedance amplifier 18, wherein the differential currents are now converted to amplified, differential voltages, denoted V.sub.o+ and V.sub.o-. Again, in FIG. 2, only the "positive" voltage signal V.sub.o+ is depicted. The alternating polarity, differential output voltage signals developed by the transimpedance amplifier 18, are next full-wave rectified by a voltage rectifier 20 to thereby develop a single ended, typically negative polarity, rectified voltage signal V.sub.R, as indicated in the exemplary waveform diagram of FIG. 2. The rectified output voltage V.sub.R is now single-ended with all negative-going peaks and exhibits an oscillation frequency twice that of the original alternating polarity differential input voltage signals V.sub.in+ or V.sub.in-.
As is well known in the art, the signal amplification associated with the gain stage 10 is defined as the product of the amplification provided by each of the amplifier stages; the transconductance amplifier 16 (with gain expressed as gm), and the transimpedance amplifier 18 (with gain expressed as R). Thus, the relationship between the positive-going peak developed through the transimpedance amplifier 18 (V.sub.o+) and the corresponding positive-going peak of the input voltage waveform (V.sub.in+) can be expressed as V.sub.o+ /V.sub.in+ =(gm.times.R), or V.sub.o+ =(gm.times.R)V.sub.in+.
A zero crossing detector (ZCD) 22 is configured, in the illustrated embodiment of FIG. 1, as a conventional switch with its inputs coupled to both of the alternating polarity differential voltage inputs 12 and 14 of the gain stage 10. The zero crossing detector 22 functions to provide a digital logical output signal (essentially a clock signal) whose transition edges define a timing interval having a periodicity exactly equal to the periodicity of the alternating polarity input voltage signal. ZCD 22 functions to mark in time those points at which the input voltage crosses the zero reference in both the negative-going and positive-going directions. The ZCD output signal provides a timing reference to the voltage rectifier 20 in order to align the resultant rectified waveform peaks.
However, this prior art-type approach to gain stage amplification and rectification is subject to both peak magnitude modulation caused by non-random noise and random error sources, and peak displacement caused by an inherent lack of symmetry in the positive and negative amplification portions of the amplifiers as well as non-symmetrical behavior in the ZCD 22 in connection with the timing edges associated to the negative-going and positive-going zero crossings. These inherent non-symmetrical response characteristics of the gain stage and ZCD cause the V.sub.R peaks to be displaced, in time, from their desired positions so as to exhibit an uncharacteristically high or low magnitude value when evaluated inside a timing window in, for example, an area integrator circuit. As depicted in general form in the timing and waveform diagram of FIG. 2, peak displacement of the amplified and rectified output signal V.sub.R can result in area cut-off of the leading or trailing edges of the waveform or can result in significant dead areas between peaks, both resulting in errors in the integrated area calculation. Minimization of these timing errors usually requires use of costly, high precision components in both the gain stage and ZCD circuits, as well as in the Integrator circuitry, leading to a significantly higher cost for a servo system demodulator circuit.